Despite the recent declines in incidence Pneumocystis carinii pneumonia (PCP) remains the leading cause of mortality among those individuals diagnosed with AIDS. Although antimicrobial compounds are available for treatment and prophylaxis, many individuals fail to respond to treatment, develop infections in spite of prophylaxis, or have serious toxicity from the available medications. Controlling the source of infection could lead to lower PCP incidence. However, due to research limitations from the lack of an in vitro culture system for Pneumocystis carinii (PC), the source of transmission remains unknown. Previously, the prevailing assumption was that reactivation of a latent infection with PC occurs in severely immunosuppressed individuals. However, recent studies indicate that PC has a limited persistence in immunocompetent persons implying that PCP results from de novo acquisition of the organism during periods of immunosuppression. Furthermore, animal studies have demonstrated airborne transmission of PC between immunosuppressed rodents, and clinical studies have suggested airborne patient-to-patient transmission. The Investigator proposes to determine if the horizontal transmission of PC from infected to susceptible humans is possible and if this mode of transmission contributes significantly more than other possible airborne environmental sources to the occurrence of PCP in susceptible persons. In order to accomplish these goals the Investigator plans to use a recently developed molecular assay that detects human PC DNA as well as a typing system that can track strain types from patient to environment. Further, the Investigator proposes to develop and validate an RT-PCR assay to assess the viability of PC organisms once collected from the environment. Using these molecular tools, the researchers will characterize the occurrence of viable human PC in the home and hospital environment, identify potential sources, and determine the influence of seasonal factors and indoor climate conditions on the presence, persistence, and viability of this organism. The long-term objective of this research is to prevent the transmission of PC in the health care as well as the home setting, thereby reducing the risk of PC infection among immunosuppressed individuals.